build/roms: Support using U-Boot as a coreboot payload

This enables embedding U-Boot into the coreboot roms as the payload. For
now, the ELF file generated by enabling CONFIG_REMAKE_ELF is used, which
includes the U-Boot binary and the board-specific device-tree file. It
might be better to use the FIT payload support for U-Boot, but that was
reportedly broken and is not tested yet.

Coreboot boards can specify payload_uboot="y" in their board.cfg to
enable building a rom with U-Boot as the payload, which is built from
the U-Boot board with the same name. Boards can further specify a
uboot_config option, to choose which board-specific config file U-Boot
should be built with.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>